Automatic Similar Thread Search is a feature that we found a long time ago that is spotty mod, due to it requires the use of a JQuery controller file. What it does, is it uses the search functions to search for similar threads made when you enter a title for a new thread and displays the top X amount of threads that matches closely before the post is submitted or really, even made. The major difference between this feature and the search was that this feature was more optimized and worked much better than the built-in search feature. Due to this optimization, it is more reliable and quicker in finding similar threads based upon the thread Title only. Although I do think that XenForo's search feature is much better than vBulletin's search feature currently.